Force sensor baseline calibration

ABSTRACT

Systems, methods, and apparatus for force sensor baseline calibration are disclosed herein. 1. Apparatus may include a force sensor configured to receive a plurality of force signals from a plurality of force sensitive elements, where the plurality of force signals is associated with a first touch at a first location of a sensing surface. The apparatus may include a touch sensor configured to receive a touch signal associated with the first touch. The apparatus may include processing logic coupled to the force sensor and the touch sensor, the processing logic being configured to determine a magnitude of a first component force associated with the first touch based, at least in part, on the plurality of force signals and the touch signal. The first component force may characterize a force applied by the first touch at the first location of the sensing surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/699,741, filed on Feb. 3, 2010, which is incorporated by referenceherein in its entirety.

TECHNICAL FIELD

This disclosure relates to the field of user interface devices and, inparticular, to determining forces applied by each of one or more touchesat a touch sensing surface.

BACKGROUND

Computing devices, such as notebook computers, personal data assistants(PDAs), kiosks, and mobile handsets, have user interface devices, whichare also known as human interface devices (HID). One user interfacedevice that has become more common is a touch-sensor pad (also commonlyreferred to as a touchpad). A basic notebook computer touch-sensor pademulates the function of a personal computer (PC) mouse. A touch-sensorpad is typically embedded into a PC notebook for built-in portability. Atouch-sensor pad replicates mouse X/Y movement by using two defined axeswhich contain a collection of sensor elements that detect the positionof a conductive object, such as a finger. Mouse right/left button clickscan be replicated by two mechanical buttons, located in the vicinity ofthe touchpad, or by tapping commands on the touch-sensor pad itself. Thetouch-sensor pad provides a user interface device for performing suchfunctions as positioning a pointer, or selecting an item on a display.These touch-sensor pads may include multi-dimensional sensor arrays fordetecting movement in multiple axes. The sensor array may include aone-dimensional sensor array, detecting movement in one axis. The sensorarray may also be two dimensional, detecting movements in two axes.

Another user interface device that has become more common is a touchscreen. Touch screens, also known as touchscreens, touch panels, ortouchscreen panels, are transparent display overlays which are typicallyeither pressure-sensitive (resistive or piezoelectric),electrically-sensitive (capacitive), acoustically-sensitive (surfaceacoustic wave (SAW)) or photo-sensitive (infra-red). The effect of suchoverlays allows a display to be used as an input device, removing thekeyboard and/or the mouse as the primary input device for interactingwith the display's content. Such displays can be attached to computersor, as terminals, to networks. Touch screens have become familiar inretail settings, on point-of-sale systems, on ATMs, on mobile handsets,on kiosks, on game consoles, and on PDAs where a stylus is sometimesused to manipulate the graphical user interface (GUI) and to enter data.A user can touch a touch screen or a touch-sensor pad to manipulatedata. For example, a user can apply a single touch, by using a finger topress the surface of a touch screen, to select an item from a menu.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings.

FIG. 1 illustrates a block diagram of an embodiment of an electronicsystem that processes touch sensor and force sensor data.

FIG. 2 illustrates a block diagram of an embodiment of an electronicsystem that processes touch sensor data.

FIG. 3 illustrates an embodiment of a touch sensing surface thatincludes force-sensing resistors.

FIGS. 4A-4C are graphs illustrating signals associated with a singletouch and release at a sensing surface, according to an embodiment.

FIGS. 5A-5C are graphs illustrating signals associated with a singletouch and release at a sensing surface, according to an embodiment.

FIGS. 6A-6C are graphs illustrating signals associated with multipletouches and releases at a sensing surface, according to an embodiment.

FIGS. 7A-7C are graphs illustrating signals associated with multipletouches and releases at a sensing surface, according to an embodiment.

FIG. 8 illustrates an embodiment of a touch sensing surface.

FIG. 9 illustrates an embodiment of a touch sensing surface with forceapplied at two touch locations.

FIG. 10 illustrates an embodiment of a touch sensing surface with forceapplied at three touch locations.

FIG. 11 is a flow diagram illustrating a force detection process,according to an embodiment.

FIG. 12 is a flow diagram illustrating a component force determiningprocess, according to an embodiment.

DETAILED DESCRIPTION

The following description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent invention. It will be apparent to one skilled in the art,however, that at least some embodiments of the present invention may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented in asimple block diagram format in order to avoid unnecessarily obscuringthe present invention. Thus, the specific details set forth are merelyexemplary. Particular implementations may vary from these exemplarydetails and still be contemplated to be within the spirit and scope ofthe present invention.

In an embodiment, an electronic system includes a touch sensing surface.This sensing surface may include touch sensors for detecting a presenceand location of one or more touches at the sensing surface, such as anarray of capacitive sensor elements. The sensing surface may alsoinclude a force sensor that determines a magnitude of force applied bythe one or more touches at the sensing surface. The force sensor mayinclude a set of force sensitive elements, such as force sensingresistors (FSRs). The electronic system may also include processinglogic that receives signals from the touch sensors and force sensor and,based on these signals, calculates a component force associated witheach of the touches at the sensing surface.

An embodiment of such an electronic system, which is capable ofcalculating multiple touch forces, is programmed as an input device thatallows a user to perform complex gestures based on single or multipleforces applied, the locations or motions of single or multiple touches,or a combination of such locations, motions, and forces. For example,such a system may be able to give a user haptic feedback proportional toforce applied, disregard false touches below a force threshold(particularly in an on-screen keyboard application), and replace on/offbuttons or other buttons.

In an embodiment of an electronic system having a force sensor, theaccuracy of the measured forces may be affected by material relaxation,hysteresis, aging effects, temperature drift, and other error sources.For example, false touch triggering may occur if the level of the rawsignal increases to near or above a touch detection threshold as aresult of error in the force sensor.

In an embodiment, a force sensor begins in an initial state, has a forceapplied to it by a touch, and because of the mechanical nature of theforce sensor, returns to a second state that is not the same as theinitial state after the force of the touch is removed. Thus, the forcesensor registers a different raw force signal after the touch is removedas compared to before the touch is applied. If this second raw forcesignal exceeds a touch detection threshold, the system may notaccurately detect the removal of the touch. Furthermore, even if theremoval of the touch is detected, the likelihood of false touches beingregistered may increase if the second raw force signal is near the touchdetection threshold.

In an embodiment, a system for detecting a force applied by one or moretouches compares the raw force signal generated by a force sensor with abaseline measurement of the force sensor. The difference between the rawforce signal and the baseline measurement is a relative force signal,which represents the actual force applied to the sensing surface by theone or more touches.

The system may also include a touch sensor, which may include an arrayof capacitive sensor elements, to detect the presence or absence of theone or more touches from the sensing surface. When no touches aredetected at the sensing surface by the touch sensor, processing logic inthe system updates the baseline measurement. In an embodiment, theprocessing logic is configured to update the baseline measurement bysetting the current raw force signal, acquired at a time when no touchis present at the sensing surface, as the baseline measurement.

Using this technique, the baseline measurement is updated so that theforce applied to the sensing surface by subsequent touches can beaccurately measured relative to the new baseline measurement, even ifthe raw force signal does not return to its previous state after thefirst touch is removed.

In an embodiment, the relative force is determined from the differencebetween the baseline measurement and the total force measured by theforce sensor. Once the relative force is determined, the magnitude andcentroid location of the relative force and the locations of multipletouches, as determined by the touch sensor, can be used to calculate acomponent force for each of the multiple touches. In an embodiment, therelative force represents a force (relative to the baseline measurement)that is applied collectively by all of the touches contacting thesensing surface, while a component force represents a force applied by asingle touch, at a single location. A relative force may also be a force(relative to the baseline measurement) that is measured by one of anumber of force sensitive elements of a force sensor.

In an embodiment, the component forces are determined using the centroidlocation of the relative force, where the relative force represents thetotal relative force measured by the force sensor. The centroiddesignates a point on the sensing surface where the force applied to thesensing surface is centered. The centroid may be envisioned as the“center of weight” of the forces applied to the sensing surface. Forexample, if the force on the sensing surface is applied at only onepoint, the centroid is at the same location as the point where the forceis applied. If the force is distributed evenly over the entire sensingsurface, the centroid is in the geometric center of the surface.

In an embodiment, the component forces are determined using a system ofequations, such as a system of equations derived from a lever balanceequation, that relates the locations of the multiple touches to thelocation and magnitude of the centroid.

FIG. 1 illustrates a block diagram of an embodiment of an electronicsystem 100 including processing logic 102 that is configured todetermine component forces for a number of multiple touches based on abaseline measurement that is updated upon detecting the absence of oneor more touches at a sensing surface. The electronic device 100 includesa touch-sensing surface 116 (e.g., a touchscreen, or a touch pad)coupled to a processing device 110 and a host 150. In an embodiment, thetouch-sensing surface 116 is a two-dimensional user interface that usesa sensor array 121 to detect touches on the surface 116. Electronicdevice 100 also includes a force sensor 104 connected to an array offorce sensitive elements 122(1)-122(N). In an embodiment, the forcesensitive elements 122(1)-122(N) are force-sensing resistors (FSRs). Inan embodiment, force sensor 104 is connected with force sensitiveelements 122(1)-122(N) through analog bus 117.

In an embodiment, the sensor array 121 includes sensor elements121(1)-121(N) (where N is a positive integer) that are disposed as atwo-dimensional matrix also referred to as an XY matrix). The sensorarray 121 is coupled to pins 113(1)-113(N) of the processing device 110via an analog bus 115 transporting multiple signals. In this embodiment,each sensor element 121(1)-121(N) is represented as a capacitor. Thecapacitance of each sensor in the sensor array 121 is measured by acapacitance sensor 101 in the processing device 110.

In an embodiment, the capacitance sensor 101 may include a relaxationoscillator or other means to convert a capacitance into a measuredvalue. The capacitance sensor 101 may also include a counter or timer tomeasure the oscillator output. The capacitance sensor 101 may furtherinclude software components to convert the count value (e.g.,capacitance value) into a sensor element detection decision (alsoreferred to as switch detection decision) or relative magnitude. Itshould be noted that there are various known methods for measuringcapacitance, such as current versus voltage phase shift measurement,resistor-capacitor charge timing, capacitive bridge divider, chargetransfer, successive approximation, sigma-delta modulators,charge-accumulation circuits, field effect, mutual capacitance,frequency shift, or other capacitance measurement algorithms. It shouldbe noted however, instead of evaluating the raw counts relative to athreshold, the capacitance sensor 101 may be evaluating othermeasurements to determine the user interaction. For example, in thecapacitance sensor 101 having a sigma-delta modulator, the capacitancesensor 101 is evaluating the ratio of pulse widths of the output,instead of the raw counts being over a certain threshold.

In an embodiment, the processing device 110 further includes processinglogic 102. Operations of the processing logic 102 may be implemented infirmware; alternatively, it may be implemented in hardware or software.The processing logic 102 may receive signals from the capacitance sensor101, and determine the state of the sensor array 121, such as whether anobject (e.g., a finger) is detected on or in proximity to the sensorarray 121 (e.g., determining the presence of the object), where theobject is detected on the sensor array, tracking the motion of theobject, or other information related to an object detected at the touchsensor.

In an embodiment, processing logic 102 also receives from force sensor104 a force signal indicating forces detected by force sensitiveelements 122(1)-122(N). The force sensitive elements 122(1)-122(N) maybe positioned to detect forces applied to touch sensing surface 116. Forexample, the force sensitive elements 122(1)-122(N) may be positioned atthe corners of the sensing surface 116 or interspersed throughout thesurface 116. The processing logic 102 may perform calculations based onthe force signal, such as determining a location of a force centroidbased on the individual forces applied to each of force sensitiveelements 122(1)-122(N), or calculating the total force appliedcollectively to all of the force sensitive elements 122(1)-122(N). In anembodiment, the force sensitive elements 122(1)-122(N) may beimplemented with force sensing devices such as FSRs, piezoelectric forcesensors, force sensitive capacitance, or another type of transducer thatconverts force to an electrical signal.

In an embodiment, the processing logic 102 determines component forcesfor multiple touches using a baseline measurement. In an embodiment, theprocessing logic 102 also manages the updating of the baselinemeasurement upon detecting the absence of one or more touches at sensingsurface 116.

In another embodiment, instead of performing the operations of theprocessing logic 102 in the processing device 110, the processing device110 may send the raw data or partially-processed data to the host 150.The host 150, as illustrated in FIG. 1, may include decision logic 151that performs some or all of the operations of the processing logic 102.Operations of the decision logic 151 may be implemented in firmware,hardware, software, or a combination thereof. The host 150 may include ahigh-level Application Programming Interface (API) in applications 152that perform routines on the received data, such as compensating forsensitivity differences, other compensation algorithms, baseline updateroutines, start-up and/or initialization routines, interpolationoperations, or scaling operations. The operations described with respectto the processing logic 102 may be implemented in the decision logic151, the applications 152, or in other hardware, software, and/orfirmware external to the processing device 110. In some otherembodiments, the processing device 110 is the host 150.

In another embodiment, the processing device 110 may also include anon-sensing actions block 103. This block 103 may be used to processand/or receive/transmit data to and from the host 150. For example,additional components may be implemented to operate with the processingdevice 110 along with the sensor array 121 (e.g., keyboard, keypad,mouse, trackball, LEDs, displays, or other peripheral devices).

The processing device 110 may reside on a common carrier substrate suchas, for example, an integrated circuit (IC) die substrate, or amulti-chip module substrate. Alternatively, the components of theprocessing device 110 may be one or more separate integrated circuitsand/or discrete components. In an embodiment, the processing device 110may be the Programmable System on a Chip (PSoC™) processing device,developed by Cypress Semiconductor Corporation, San Jose, Calif.Alternatively, the processing device 110 may be one or more otherprocessing devices known by those of ordinary skill in the art, such asa microprocessor or central processing unit, a controller,special-purpose processor, digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA), or other programmable device. In an alternativeembodiment, for example, the processing device 110 may be a networkprocessor having multiple processors including a core unit and multiplemicro-engines. Additionally, the processing device 110 may include anycombination of general-purpose processing device(s) and special-purposeprocessing device(s).

In an embodiment, the electronic system 100 is implemented in a devicethat includes the touch-sensing surface 116 as the user interface, suchas handheld electronics, portable telephones, cellular telephones,notebook computers, personal computers, personal data assistants (PDAs),kiosks, keyboards, televisions, remote controls, monitors, handheldmulti-media devices, handheld video players, gaming devices, controlpanels of a household or industrial appliances, or other computerperipheral or input devices. Alternatively, the electronic system 100may be used in other types of devices. It should be noted that thecomponents of electronic system 100 may include all the componentsdescribed above. Alternatively, electronic system 100 may include onlysome of the components described above, or include additional componentsnot listed herein.

FIG. 2 is a block diagram illustrating an embodiment of a capacitivetouch sensor array 121 and a capacitance sensor 101 that convertsmeasured capacitances to coordinates. In an embodiment, sensor array 121and capacitance sensor 101 are implemented in a system such aselectronic system 100. Sensor array 220 includes a matrix 225 of N×Melectrodes (N receive electrodes and M transmit electrodes), whichfurther includes transmit (TX) electrode 222 and receive (RX) electrode223. Each of the electrodes in matrix 225 is connected with capacitancesensing circuit 201 through multiplexers 212 and 213.

Capacitance sensor 101 includes multiplexer control 211, multiplexers212 and 213, clock generator 214, signal generator 215, demodulationcircuit 216, and analog to digital converter (ADC) 217. ADC 217 isfurther coupled with touch coordinate converter 218. Touch coordinateconverter 218 outputs a signal to the processing logic 102.

The transmit and receive electrodes in the electrode matrix 225 may bearranged so that each of the transmit electrodes overlap and cross eachof the receive electrodes such as to form an intersection, whilemaintaining galvanic isolation from each other. Thus, each transmitelectrode may be capacitively coupled with each of the receiveelectrodes. For example, transmit electrode 222 is capacitively coupledwith receive electrode 223 at the point where transmit electrode 222 andreceive electrode 223 intersect.

Clock generator 214 supplies a clock signal to signal generator 215,which produces a TX signal 224 to be supplied to the transmit electrodesof touch sensor 220. In an embodiment, the signal generator 215 includesa set of switches that operate according to the clock signal from clockgenerator 214. The switches may generate a TX signal 224 by periodicallyconnecting the output of signal generator 215 to a first voltage andthen to a second voltage, wherein said first and second voltages aredifferent.

The output of signal generator 215 is connected with multiplexer 212,which allows the TX signal 224 to be applied to any of the M transmitelectrodes of touch sensor 220. In an embodiment, multiplexer control211 controls multiplexer 212 so that the TX signal 224 is applied toeach transmit electrode in a controlled sequence. Multiplexer 212 mayalso be used to ground, float, or connect an alternate signal to theother transmit electrodes to which the TX signal 224 is not currentlybeing applied.

Because of the capacitive coupling between the transmit and receiveelectrodes, the TX signal 224 applied to each transmit electrode inducesa current within each of the receive electrodes. For instance, when theTX signal 224 is applied to transmit electrode 222 through multiplexer212, the TX signal 224 induces an RX signal 227 on the receiveelectrodes in matrix 225. The RX signal 227 on each of the receiveelectrodes can then be measured in sequence by using multiplexer 213 toconnect each of the N receive electrodes to demodulation circuit 216 insequence.

The capacitance associated with each intersection between a TX electrodeand an RX electrode can be sensed by selecting every availablecombination of TX electrode and an RX electrode using multiplexers 212and 213. To improve performance, multiplexer 213 may also be segmentedto allow more than one of the receive electrodes in matrix 225 to berouted to additional demodulation circuits 216. In an optimizedconfiguration, wherein there is a 1-to-1 correspondence of instances ofdemodulation circuit 216 with receive electrodes, multiplexer 213 maynot be present in the system.

When an object, such as a finger, approaches the electrode matrix 225,the object causes a decrease in the mutual capacitance between only someof the electrodes. For example, if a finger is placed near theintersection of transmit electrode 222 and receive electrode 223, thepresence of the finger will decrease the mutual capacitance between thetwo electrodes 222 and 223. Thus, the location of the finger on thetouchpad can be determined by identifying the one or more receiveelectrodes having a decreased mutual capacitance in addition toidentifying the transmit electrode to which the TX signal 224 wasapplied at the time the decreased mutual capacitance was measured on theone or more receive electrodes.

By determining the mutual capacitances associated with each intersectionof electrodes in the matrix 225 the locations of one or more contactsmay be determined. The determination may be sequential, in parallel, ormay occur more frequently at commonly used electrodes.

In alternative embodiments, other methods for detecting the presence ofa finger or conductive object may be used where the finger or conductiveobject causes an increase in capacitance at one or more electrodes,which may be arranged in a grid or other pattern. For example, a fingerplaced near an electrode of a capacitive sensor may introduce anadditional capacitance to ground that increases the total capacitancebetween the electrode and ground. The location of the finger can bedetermined from the locations of one or more electrodes at which anincreased capacitance is detected.

The induced current waveform 227 is rectified by demodulation circuit216. The rectified current output by demodulation circuit 216 can thenbe filtered and converted to a digital code by ADC 217.

The digital code is converted to touch coordinates indicating a positionof an input on touch sensor array 121 by touch coordinate converter 218.The touch coordinates are transmitted as an input signal to theprocessing logic 102.

In an embodiment, the sensor array 121 can be configured to detectmultiple touches. One technique for multi-touch detection uses atwo-axis implementation: one axis to support rows and another axis tosupport columns. Additional axes, such as a diagonal axis, implementedon the surface using additional layers, can allow resolution ofadditional touches.

Another method for multi-touch detection has the user insert a timedelay between the first and subsequent touches on the touch-sensingsurface.

In an embodiment of a system that supports multiple touch detection,processing logic 102 may receive touch coordinates for each of a numberof simultaneous touches at the sensing surface. The processing logic 102may further use these touch coordinates to determine factors such as thenumber of touches present at the sensing surface, or may use the touchcoordinates when calculating a component force for each of the touches.

FIG. 3 illustrates an embodiment of a touch sensing surface 116 thatincludes force sensitive elements 122(1)-122(4). In an embodiment, touchsensing surface 116 may be layered over an array of capacitive sensorelements similar to sensor array 121. In an embodiment, capacitivesensing techniques using mutual capacitance measurement, such as thecapacitive sensing techniques described with reference to FIGS. 1 and 2,may be used to measure capacitances of the capacitive sensor elementswhile measuring forces using force sensitive elements, such as forcesensitive elements 122(1)-122(4). Alternatively, force sensitiveelements may be used in a single-touch sensing system, pseudo-multitouchsystem, or other capacitive touch sensing system that operates bymeasuring self-capacitance of capacitive sensor elements.

Touch sensing surface 116 and force sensitive elements 122(1)-122(4) maybe connected with a processing device 110 as described above withreference to FIG. 1.

In an embodiment, each of force sensitive elements 122(1)-122(4) has aninitial resistance that changes in response to force applied to theforce sensitive element. For example, an force sensitive element mayinclude a conductor that deforms in response to strain or stress,changing the resistance of the conductor.

In an embodiment, the sensing surface 116 is rectangular and the forcesensitive elements 122(1)-122(4) are placed at the corners of thesensing surface 116. In alternative embodiments, devices used toimplement force sensitive elements may include piezoelectric sensors,force sensitive capacitances, or any other devices that can convertapplied force to an electrical signal.

In an embodiment, signals from the force sensitive elements122(1)-122(4) are used to calculate a magnitude and a location of aforce centroid for the force applied by multiple touches at the sensingsurface 116. The force centroid location indicates the “center ofweight” of the applied force. The calculations for determining themagnitude of force and the location of the force centroid may beperformed, for example, in processing logic 102 of processing device 110or in host 150.

In an embodiment, the magnitude of the total force applied to thesensing surface 116 by the multiple touches is proportional to the sumof forces applied to each of the force sensitive elements 122(1)-122(4).For example, the processing logic 102 may calculate the magnitude byadding the individual forces measured by each of the force sensitiveelements 122(1)-122(4). Alternatively, the processing logic 102 maycalculate the magnitude as an average of the individual forces measuredby each of the force sensitive elements 122(1)-122(4). The magnitude maybe calculated similarly for other systems that have fewer or more thanfour force sensitive elements, or use other types of sensors in place offorce sensitive elements.

In an embodiment, the magnitudes of the individual forces measured byeach force sensitive element or sensor are determined relative to abaseline measurement. For example, a force sensitive element may be usedto generate a raw force signal from which the baseline measurement issubtracted to generate a relative force signal for the force sensitiveelement. Where the baseline measurement represents the raw force signalthat is generated by the force sensitive element when no force isapplied to the force sensitive element, the relative force signal forforce sensitive element more accurately represents the force applied tothe force sensitive element.

In an alternative embodiment, the total force signal may be calculated(i.e., by summing or averaging the raw force sensitive element forcesignals), and the difference between the total force signal and thebaseline measurement may be calculated to generate a total relativeforce signal.

In an embodiment, the baseline measurement is updated according to arecalibration scheme to compensate for errors due to the physical natureof the force sensitive element, which results in a raw force signal thatdoes not return to its previous state after a touch is lifted from thesensing surface. This effect is illustrated in FIGS. 4A, 4B, and 4C.

FIGS. 4A-4C are graphs illustrating signals associated with a singletouch and release at a sensing surface, according to an embodiment of anelectronic system where baseline recalibration is not enabled. Thegraphs in FIG. 4 plot changes in these signals over time, inmilliseconds.

With reference to FIG. 4A, the raw force signal 401 represents a signalfrom one force sensitive element or may also correspond to an average orsum of more than one force sensitive element. In an embodiment whereanother type of force sensing device is used, such as a piezoelectricforce sensor or force sensitive capacitance, the signal from the othertype of force sensing device may have similar characteristics as the rawforce signal 401.

The raw force signal 401 has a rising edge and a falling edge, whichcorrespond to a touch being placed on the sensing surface andsubsequently being removed from the sensing surface, respectively.Baseline measurement 402 ideally indicates the level of the raw forcesignal 401 when no touch is present, however, the raw force signal 401does not return to its previous state after the touch is lifted. Thus,the raw force signal 401 is significantly higher than the baselinemeasurement 402 even after the touch is lifted from the sensing surface.

With reference to FIG. 4B, touch detector signal 403 is a signalrepresenting whether a touch is present or absent from the sensingsurface. In an embodiment, the touch detector signal 403 may begenerated based on a signal from a touch sensor, such as an array ofcapacitive sensor elements, or a surface acoustic wave touch sensingdevice. For example, in an electronic system having a capacitive touchsensor, the touch detector signal may be high when the capacitance ofthe touch sensor exceeds a threshold value, indicating a touch at thesensing surface.

With reference to FIG. 4C, the relative force signal 404 represents thedifference between the raw force signal 401 and the baseline measurement402. The relative force signal 404 is ideally an accurate representationof the force applied to the force sensitive element, since it removesthe offset attributable to the baseline force signal, which is the rawforce signal when no touch is present at the sensing surface. However,since the raw force signal does not return to its previous state afterthe touch is removed, the relative force signal 404 has an offset afterthe touch is removed.

In an embodiment, the electronic system may perform some action based onwhether the relative force signal 404 exceeds a force threshold 405. Ina situation where the updating of the baseline measurement 402 is notenabled, the relative force signal 404 may continue to exceed the forcethreshold 405 even after a touch is removed from the sensing surface.Thus, such an electronic system may erroneously perform the actionassociated with the force threshold 405 even when the touch is absentfrom the sensing surface.

FIGS. 5A, 5B, and 5C are graphs illustrating signals associated with asingle touch and release at a sensing surface, according to anembodiment of an electronic system implementing baseline recalibration.

With reference to FIG. 5A, in an embodiment of an electronic system inwhich recalibration by updating the baseline measurement 502 is enabled,the system includes processing logic, such as processing logic 102,which manages recalibration of the baseline measurement 502. Theprocessing logic 102 performs the update of the baseline measurementbased on the raw force signal 501, and the touch detector signal 503.

With reference to FIG. 5B, in an embodiment, the processing logicdetects the falling edge 513 of touch detector signal 503, whichcorresponds to removal of a touch from the sensing surface, and updatesthe baseline measurement 502 in response to detecting the falling edge513. The update of baseline measurement 502 occurs at the baselineupdate time 512. In an embodiment, the baseline update time 512 is thetime at which the falling edge 513 is detected. Alternatively, thebaseline update 512 may be delayed with respect to the falling edge 513.

With reference to FIG. 5C, the processing logic 102 may update thebaseline measurement 502 by measuring the raw force signal and storingthe measurement in a memory as a new baseline measurement. Theprocessing logic 102 subsequently accesses the stored measurement whencalculating the relative force signal 504.

Thus, even though the signal level of the raw force signal 501 does notreturn to its previous pre-touch state, the new baseline measurementcompensates for the difference in the post-touch offset. The relativeforce signal 504, which represents the difference between the raw forcesignal 501 and the baseline measurement 502, more accurately representsthe force applied to the sensing surface after the touch has beenlifted. In particular, the relative force signal 504 accuratelyindicates that no force is being applied to the sensing surface afterremoval of the touch.

In an embodiment where the electronic system performs an action based onthe relative force signal 504, the action may be triggered when therelative force signal exceeds a force threshold 505. In a system whererecalibration by updating the baseline measurement is enabled, theaction associated with the force threshold 505 is not erroneouslyperformed as a result of the relative force signal 504 exceeding theforce threshold 505 when no touch is present at the sensing surface.

In an embodiment, the baseline recalibration allows the electronicsystem to ignore non-capacitive touches at the sensing surface. Forexample, in an embodiment, the touch sensor is a capacitive touchsensor. If the touch sensor does not detect a touch at the sensingsurface, the relative force signal is zero even when a non-capacitiveforce is applied to the sensing surface. Thus, a force applied to thetouch sensing surface is not registered as a touch unless it is alsodetected by the touch sensor.

FIGS. 6A, 6B, and 6C are graphs illustrating signals associated withmultiple touches and releases at a sensing surface, according to anembodiment of an electronic system where baseline recalibration is notenabled.

With reference to FIG. 6A, the raw force signal 601 changes in responseto three touches that are applied to and lifted from the sensingsurface. After each touch, the raw force signal 601 returns to adifferent baseline. The baseline measurement 602, however, is notupdated. With reference to FIG. 6B, touch detector signal 603 changes inresponse to the same three touches, and more clearly indicates thebeginning and end of each touch, as compared to the raw force signal601.

With reference to FIG. 6C, the relative force signal 604 represents thedifference between the raw force signal 601 and the baseline measurement602. In an embodiment, the electronic system is configured to perform anaction when the relative force signal 604 exceeds force threshold 605.Without the baseline recalibration, the relative force signal 604exceeds the force threshold 605 after the initial touch, even after thetouch is removed. The force threshold 605 does not detect either of thesubsequent touches. In an embodiment of an electronic system where theforce threshold 605 is associated with an action to be performed oncefor every touch, the electronic system would erroneously fail to performthe action once for each of the three touches.

For example, the sensing surface may be used to control a cursor in adrawing program, where the size of the cursor changes between two ormore sizes according to the force applied to the sensing surface. Inthis example, the force threshold 605 may determine when the cursor sizeincreases to the next size. Thus, without the baseline recalibration,the cursor changes size once when it ideally should have changed to thenext size and back three times, in response to three touches.

FIGS. 7A, 7B, and 7C are graphs illustrating signals associated withmultiple touches and releases at a sensing surface, according to anembodiment of an electronic system where baseline recalibration isenabled.

In FIG. 7A, the baseline measurement 702 is updated three times,starting at baseline update 712. The three updates of baselinemeasurement 702 occur in response to three falling edges of touchdetector signal 703 of FIG. 7B, which indicate that a touch has beenlifted from the sensing surface.

With regard to FIG. 7C, the relative force signal 704 represents thedifference between the raw force signal 701 and the baseline measurement702. Because of the updates of baseline measurement 702, the relativeforce signal more accurately indicates the magnitude of the forceapplied by each of the multiple touches at the sensing surface, ascompared to relative force signal 604, where baseline recalibration isnot enabled.

Force threshold 705, which may be associated with an action aspreviously described, is exceeded three times by relative force signal704, in accord with the three touches indicated by touch detector signal303.

In an embodiment, the baseline recalibration is performed separately foreach of the force sensitive elements or other force sensing devices inthe touch sensing surface. Using the relative force signal from each ofthe force sensitive elements, the processing logic 102 may determine aposition of a force centroid, which indicates the “center of weight” ofthe force applied to the sensing surface. For example, if a force isapplied at the sensing surface closer to a first force sensitive elementthan to a second force sensitive element, the first force sensitiveelement will measure a greater force than the second force sensitiveelement. Thus, based on the weighting of forces measured by each of theforce sensitive, the processing logic can determine the centroidlocation.

In an embodiment, the raw force signal may be used to detect one or morenon-capacitive touches at the sensing surface. For example, theprocessing logic may update the baseline measurement in response todetecting that the raw force signal is below a threshold level. In anembodiment, the update of the baseline measurement may be implementedusing a low-pass filter (LPF), such as an infinite impulse response(IIR) or finite impulse response (FIR) filter. When the raw force signalexceeds the threshold, the baseline measurement is not updated. Thus,the relative force signal is proportional to the force component of thenon-capacitive touch. In an embodiment, the updating of the baselinemeasurement may be periodic.

The periodic updating of the baseline measurement may also be used incombination with updating of the baseline measurement in response todetecting the removal of one or more touches at the touch sensingsurface. This combination scheme may be used, for example, whendetecting both capacitive and non-capacitive touches at the touchsensing surface.

FIG. 8 illustrates an embodiment of a rectangular touch sensing surface116 having a force sensitive element at each of its corners. Locationson touch sensing surface 116 are designated by Cartesian coordinatesaccording to X-axis 821 and Y-axis 822. With reference to FIG. 8, theprocessing logic may calculate the location of force centroid 801 usingEquations 1 and 2:

$\begin{matrix}{x = {{Size\_ x} \cdot \frac{{sB} + {sD}}{{sA} + {sB} + {sC} + {sD}}}} & \left( {{Equation}\mspace{14mu} 1} \right) \\{y = {{Size\_ y} \cdot \frac{{sC} + {sD}}{{sA} + {sB} + {sC} + {sD}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

In Equations 1 and 2, (x,y) is the location of the force centroid in aCartesian coordinate system, Size_x is the width of the touch sensingsurface 116, Size_y is the height of the touch sensing surface 116, andsA, sB, sC, and sD are magnitudes of forces measured at the forcesensitive elements 122(1)-122(4), respectively.

In an embodiment, when only one touch is present at the touch sensingsurface 116, the location of the force centroid 801 corresponds to thelocation of the touch. Thus, the processing logic may determine thelocation of the touch by calculating the touch location from the touchsignal, by calculating the location of a force centroid from relativeforce signals, or both. For example, the processing logic may determinethe location of the single touch by averaging the locations of the forcecentroid and the location of the touch as detected by the touch sensor.

The processing logic may calculate the magnitude of the force applied bythe single touch using Equation 3 below

sF=sA+sB+sC+sD   (Equation 3)

In Equation 3, sF represents the force applied by the single touch,which can be calculated as the sum of the forces sA, sB, sC, and sDmeasured by the force sensitive elements 122(1)-122(4).

FIG. 9 illustrates two touches on a rectangular touch sensing surface116 having one of force sensitive elements 122(1)-122(4) at each of itscorners. Locations on touch sensing surface 116 are designated byCartesian coordinates according to X-axis 821 and Y-axis 822. In anembodiment, when two touches are present at the touch sensing surface116, the component forces applied by each of the touches to the sensingsurface 116 may be calculated using the location and magnitude sF of theforce centroid, the locations of the two touches, and a lever balanceequation, which is expressed as Equation 4.

Equation 5 expresses the length L1 in terms of the Cartesian coordinatelocations of the first touch 902 at (x₁, y₁) and the force centroid at(x, y). Equation 6 expresses the length L2 in terms of the Cartesiancoordinate locations of the second touch 903 at (x₂, y₂) and the forcecentroid at (x, y).

FS1·L1=FS2·L2   (Equation 4)

L1=√{square root over ((x−x ₁)²+(y−y ₁)²)}{square root over ((x−x₁)²+(y−y ₁)²)}   (Equation 5)

L2=√{square root over ((x−x ₂)²+(y−y ₂)²)}{square root over ((x−x₂)²+(y−y ₂)²)}   (Equation 6)

Equations 4, 5, and 6 can be simplified into the system of equationsexpressed in Equations 7.

$\begin{matrix}\left\{ \begin{matrix}{{{FS}\; {1 \cdot L}\; 1} = {F\; S\; {2 \cdot L}\; 2}} \\{{{{FS}\; 1} + {{FS}\; 2}} = {{sA} + {sB} + {sC} + {sD}}}\end{matrix} \right. & \left( {{Equations}\mspace{14mu} 7} \right)\end{matrix}$

In Equations 7, FS1 represents the component force applied by the firsttouch at location (x_(1,) y₁), L1 represents the distance between thefirst touch and the force centroid at (x, y), FS2 represents thecomponent force applied by the second touch at location (x_(2,) y₂), andL2 represents the distance between the second touch and the forcecentroid. The forces detected by force sensitive elements 122(1)-122(4)are represented in Equations 7 by sA, sB, sC, and sD.

Solving the Equations 7 for the component forces FS1 and FS2 of thefirst touch and the second touch, respectively, yields Equations 8 and9.

$\begin{matrix}{{{FS}\; 1} = {{\frac{{sA} + {sB} + {sC} + {sD}}{{L\; 1}\; + {L\; 2}} \cdot ~L}\; 2}} & \left( {{Equation}\mspace{14mu} 8} \right) \\{{{FS}\; 2} = {{\frac{{sA} + {sB} + {sC} + {sD}}{{L\; 1}\; + {L\; 2}} \cdot ~L}\; 1}} & \left( {{Equation}\mspace{14mu} 9} \right)\end{matrix}$

In an embodiment, the processing logic determines the component forcesapplied by each of the two touches using Equations 8 and 9.

FIG. 10 illustrates three touches on a rectangular touch sensing surface116 having one of force sensitive elements 122(1)-122(4) at each of itscorners. Locations on touch sensing surface 116 are designated byCartesian coordinates according to X-axis 821 and Y-axis 822. In anembodiment, when three touches are present at the touch sensing surface116, the component forces applied by each of the touches to the sensingsurface 116 may be calculated using the location and magnitude sF of theforce centroid, the locations of the three touches, and a system ofequations relating these values, which is expressed in Equations 10.

$\begin{matrix}\left\{ \begin{matrix}{{{{FS}\; {1 \cdot \left( {x - x_{1}} \right)}} + {{FS}\; {2 \cdot \left( {x - x_{2}} \right)}} + {{FS}\; {3 \cdot \left( {x - x_{3}} \right)}}} = 0} \\{{{{FS}\; {1 \cdot \left( {y - y_{1}} \right)}} + {{FS}\; {2 \cdot \left( {y - y_{2}} \right)}} + {{FS}\; {3 \cdot \left( {y - y_{3}} \right)}}} = 0} \\{{{{FS}\; 1} + {{FS}\; 2} + {{FS}\; 3}} = {{sA} + {sB} + {sC} + {sD}}}\end{matrix} \right. & \left( {{Equations}\mspace{14mu} 10} \right)\end{matrix}$

In Equations 10, FS1, FS2, and FS3 represent the component forcesapplied by the first touch, the second touch, and the third touch,respectively, and (x₁, y₁), (x₂, y₂), and (x₃, x₃) represent Cartesiancoordinates locations of the first, second, and third touches,respectively. The forces measured by the force sensitive elements122(1)-122(4) are represented by sA, sB, sC, and sD.

Solving for FS1, FS2, and FS3 results in Equations 11, 12, and 13,respectively.

$\begin{matrix}{{{FS}\; 1} = {\frac{{x \cdot y_{2}} - {x_{2} \cdot y} - {x \cdot y_{3}} + {x_{3} \cdot y} + {x_{2} \cdot y_{3}} - {x_{3} \cdot y_{2}}}{{x_{1} \cdot y_{2}} - {x_{2} \cdot y_{1}} - {x_{1} \cdot y_{3}} + {x_{3} \cdot y_{1}} + {x_{2} \cdot y_{3}} - {x_{3} \cdot y_{2}}} \cdot \left( {{sA} + {sB} + {sC} + {sD}} \right)}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$

In an embodiment, the processing logic 102 may determine the componentforces FS1, FS2, and FS3 that are applied by the first, second, andthird touches at the sensing surface 116 using the relationshipsdescribed in Equations 11, 12, and 13.

In an embodiment, the Equations 11, 12, and 13 are used when the threetouches are not in a straight line. Equation 14 describes the conditionwhere the three touches, at Cartesian coordinate locations (x₁, y₁),(x₂, y₂), and (x₃, y₃) are not in a straight line.

x ₁ ·y ₂ −x ₂ ·y ₁ −x ₁ ·y ₃ +x ₃ ·y ₁ +x ₂ ·y ₃ −x ₃ ·y ₂≠0   (Equation14)

FIG. 11 is a flow diagram illustrating a force detection process 1100for determining a component force applied by each of one or more touchesto a sensing surface. In an embodiment, process 1100 may be performed byprocessing logic, such as processing logic 102. The operations ofprocess 1100 that may be performed by processing logic 102 mayalternatively be performed in host 150.

Force detection process 1100 begins at block 1101, at which theprocessing logic receives a force signal from the force sensor. Forexample, processing logic 102 may receive the force signal from forcesensor 104 at a force sensor input. In an embodiment, the force signalindicates a force measured by each of a number of force sensitiveelement, such as force sensitive elements 122(1)-122(N). From block1101, the process 1100 continues at block 1103.

At block 1103, the processing logic receives a touch signal from a touchsensor. For example, in an embodiment where the touch sensor is acapacitive sensor array 121, the processing logic 102 may receive atouch signal from capacitance sensor 101 at a touch sensor input. In anembodiment, the touch signal indicates the presence or absence of one ormore touches at the touch sensing surface. From block 1103, the process1100 continues at block 1105.

At block 1105, the processing logic determines whether the touch signalindicates the absence of one or more touches at the sensing surface. Ifthe touch signal indicates the absence of the one or more touches at thesensing surface, the process 1100 continues a block 1107.

At block 1107, the processing logic performs a baseline recalibration bysetting the baseline measurement equal to a raw force signalmeasurement, where the raw force signal measurement is acquired at atime when the one or more touches are absent from the sensing surface.For example, when no touch is present at the touch sensing surface 116,the absence of any touch is indicated in the touch signal transmittedfrom the capacitance sensor 101 to the touch signal input of theprocessing logic 102. Processing logic 102, in response to determiningfrom the touch signal that no touch is present at the sensing surface116, responds by acquiring a present measurement of the force signalfrom force sensor 104. The measurement is stored as a new baselinemeasurement. From block 1107, the process 1100 continues back to block1101.

If, at block 1105, the touch signal does not indicate that the one ormore touches are absent from the sensing surface, the process 1100continues at block 1109. At block 1109, the processing logic determinesa relative force magnitude based on the force signal and the baselinemeasurement. For example, the processing logic 102 may calculate themagnitude of the relative force signal by calculating the differencebetween the baseline measurement and a raw force signal received fromforce sensor 104. From block 1109, the process 1100 continues at block1111

At block 1111, the processing logic uses the touch signal to determinelocations of each of the one or more touches. In an embodiment, thetouch sensing surface may include a grid or other arrangement of sensorthat allows the locations of multiple touches to be determined. Forexample, the sensor array 121 includes a grid of capacitive sensorelements 121(1)-121(N) and is configured to detect locations of multipletouches applied to the sensing surface 116. The location information isindicated in the touch signal transmitted from capacitance sensor 101 toprocessing logic 102. Processing logic 102 determines the touchlocations from the received touch signal. From block 1111, the process1100 continues at block 1113.

A block 1113, the processing logic uses the touch locations and therelative force magnitude to determine a magnitude of a component forcefor each of the one or more touches at the sensing surface. In anembodiment, the processing logic may determine the number of touchesthat are being applied to the touch sensing surface. Depending on thenumber of touches, the processing logic may determine the componentforces using one of a number of operations. For example, the processinglogic may use a system of equations, a lever balance equation, or mayalternatively set the component force equal to the total relative force.The operations of block 1113 are further described with reference toFIG. 12. From block 1113, the process 1100 continues back to block 1101.

FIG. 12 is a flow diagram illustrating a component force determiningprocess 1200, according to an embodiment. In an embodiment, theoperations of process 1200 correspond to operations that may beperformed in block 1113 of FIG. 11.

The process 1200 begins at block 1201. At block 1201, the processinglogic determines the number of touches present at the touch sensingsurface from the touch signal. From block 1201, the process 1200continues at block 1203.

At block 1203, if three touches are present at the touch sensingsurface, the process 1200 continues at block 1205.

At block 1205, the processing logic uses a system of equations todetermine a component force for each of the three touches. In anembodiment, the processing logic 102 uses the system of equationsexpressed in Equations 10 to determine a component force for each of thethree touches at sensing surface 116.

If, at block 1203, three touches are not present at the sensing surface,the process 1200 continues at block 1207. At block 1207, if two touchesare present at the touch sensing surface, the process 1200 continues atblock 1209.

At block 1209, the processing logic uses a lever balance equation todetermine a component force for each of the two touches. In anembodiment, the lever balance equation and the equations for thedistance between a force centroid and touch locations (as expressed inEquation 4, 5, and 6) can be simplified to a system of equations asexpressed in Equations 7. In an embodiment, the processing logicdetermines the component force for each of the two touches using thissystem of equations.

If, at block 1207, two touches are not present at the sensing surface,the process 1200 continues at block 1211. At block 1211, if one touch ispresent at the touch sensing surface, the process 1200 continues atblock 1213.

At block 1213, the processing logic uses a relative force as thecomponent force or the single touch. For example, with reference toFIGS. 1 and 8, the processing logic 102 may calculate a force sF appliedby the single touch by summing the forces sA, sB, sC, and sD measured bythe force sensitive elements 122(1)-122(4), in accord with Equation 3.

Thus, the processing logic determines a component force for each of theone or more touches at the touch sensing surface. In an embodiment, thecomponent forces associated with each of the touches may be used toimplement functions of a user interface, and may allow a user to performactions such as changing modes or manipulate objects in a graphical userinterface. In an embodiment, the detection of component forces may alsobe used to identify false touches. For example, an on-screen keyboardapplication may use the component forces to determine whether a userintended to activate a key.

Embodiments of the present invention, described herein, include variousoperations. These operations may be performed by hardware components,software, firmware, or a combination thereof. As used herein, the term“coupled to” may mean coupled directly or indirectly through one or moreintervening components. Any of the signals provided over various busesdescribed herein may be time multiplexed with other signals and providedover one or more common buses. Additionally, the interconnection betweencircuit components or blocks may be shown as buses or as single signallines. Each of the buses may alternatively be one or more single signallines and each of the single signal lines may alternatively be buses.

Certain embodiments may be implemented as a computer program productthat may include instructions stored on a computer-readable medium.These instructions may be used to program a general-purpose orspecial-purpose processor to perform the described operations. Acomputer-readable medium includes any mechanism for storing ortransmitting information in a form (e.g., software, processingapplication) readable by a machine (e.g., a computer). Thecomputer-readable storage medium may include, but is not limited to,magnetic storage medium (e.g., floppy diskette); optical storage medium(e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM);random-access memory (RAM); erasable programmable memory (e.g., EPROMand EEPROM); flash memory, or another type of medium suitable forstoring electronic instructions.

Additionally, some embodiments may be practiced in distributed computingenvironments where the computer-readable medium is stored on and/orexecuted by more than one computer system. In addition, the informationtransferred between computer systems may either be pulled or pushedacross the transmission medium connecting the computer systems.

Although the operations of the method(s) herein are shown and describedin a particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittentand/or alternating manner.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense.

What is claimed is:
 1. An apparatus comprising: a force sensorconfigured to receive a plurality of force signals from a plurality offorce sensitive elements, wherein the plurality of force signals isassociated with a first touch at a first location of a sensing surface;a touch sensor configured to receive a touch signal associated with thefirst touch; and processing logic coupled to the force sensor and thetouch sensor, the processing logic being configured to determine amagnitude of a first component force associated with the first touchbased, at least in part, on the plurality of force signals and the touchsignal, the first component force characterizing a force applied by thefirst touch at the first location of the sensing surface.
 2. Theapparatus of claim 1, wherein the plurality of force signals is furtherassociated with a second touch at a second location of the sensingsurface, wherein the first touch and the second touch are concurrent,and wherein the processing logic is further configured to determine amagnitude of a second component force associated with the second touch.3. The apparatus of claim 2, wherein the processing logic is furtherconfigured to determine a total force associated with the first touchand the second touch, and wherein the processing logic is furtherconfigured to determine a centroid location associated with the firsttouch and the second touch.
 4. The apparatus of claim 3, wherein thefirst component force and the second component force are determinedbased, at least in part, on a lever balance equation.
 5. The apparatusof claim 4, wherein the lever balance equation is:FS1*L1=FS2*L2, and wherein FS1 characterizes the first component force,L1 characterizes a distance between the first location and the centroidlocation, FS2 characterizes the second component force, and L2characterizes a distance between the second location and the centroidlocation.
 6. The apparatus of claim 3, wherein the plurality of forcesignals is further associated with a third touch at a third location ofthe sensing surface, wherein the first touch, the second touch, and thethird touch are concurrent, and wherein the processing logic is furtherconfigured to determine a magnitude of a third component forceassociated with the third touch.
 7. The apparatus of claim 6, whereinthe processing logic is configured to determine a first magnitude of thefirst component force, a second magnitude of the second component force,and a third magnitude of the third component force based on a system ofequations.
 8. The apparatus of claim 7, wherein the system of equationsincludes a plurality of equations comprising:FS1*(x−x ₁)+FS2*(x−x ₂)+FS3*(x−x ₃)=0;FS1*(y−y ₁)+FS2*(y−y ₂)+FS3*(y−y ₃)=0;FS1+FS2+FS3=sA+sB+sC+sD; and wherein FS1 characterizes the firstcomponent force, wherein FS2 characterizes the second component force,wherein FS3 characterizes the third component, wherein (x₁, y₁)characterizes a first Cartesian coordinate location of the first touchdetermined based on the touch signal, wherein (x₂, y₂) characterizes asecond Cartesian coordinate location of the second touch determinedbased on the touch signal, wherein (x₃, y₃) characterizes a thirdCartesian coordinate location of the third touch determined based on thetouch signal, and wherein sA, sB, sC, and sD represent the plurality offorce signals received from the plurality of force sensitive elements.9. The apparatus of claim 1, wherein the plurality of force sensitiveelements includes at least three force sensitive elements.
 10. Theapparatus of claim , wherein the processing logic is further configuredto update a baseline associated with the plurality of force signals. 11.A method comprising: receiving a plurality of force signals from aplurality of force sensitive elements, wherein the plurality of forcesignals is associated with a first touch at a first location of asensing surface; receiving a touch signal associated with the firsttouch; and determining a first magnitude of a first component forceassociated with the first touch based, at least in part, on theplurality of force signals and the touch signal, the first componentforce characterizing a force applied by the first touch at the firstlocation of the sensing surface.
 12. The method of claim 11, wherein theplurality of force signals is further associated with a second touch ata second location of the sensing surface and a third touch at a thirdlocation of the sensing surface, and wherein the first touch, the secondtouch, and the third touch are concurrent.
 13. The method of claim 12further comprising: determining a total force associated with the firsttouch, the second touch, and the third touch; and determining a centroidlocation associated with the total force.
 14. The method of claim 13further comprising: determining a second magnitude of a second componentforce associated with the second touch; and determining a thirdmagnitude of a third component force associated with the third touch.15. The method of claim 11 further comprising: updating a baselineassociated with the plurality of force signals.
 16. A system comprising:a sensing surface; a plurality of force sensitive elements configured tomeasure forces associated with the sensing surface, and furtherconfigured to generate a plurality of force signals based on themeasured forces; a force sensor configured to receive the plurality offorce signals from the plurality of force sensitive elements, whereinthe plurality of force signals is associated with a first touch at afirst location of the sensing surface; a touch sensor configured toreceive a touch signal associated with the first touch; and processinglogic coupled to the force sensor and the touch sensor, the processinglogic being configured to determine a first magnitude of a firstcomponent force associated with the first touch based, at least in part,on the plurality of force signals and the touch signal, the firstcomponent force characterizing a force applied by the first touch at thefirst location of the sensing surface.
 17. The system of claim 16,wherein the plurality of force signals is further associated with asecond touch at a second location of the sensing surface, wherein thefirst touch and the second touch are concurrent, and wherein theprocessing logic is further configured to determine a magnitude of asecond component force associated with the second touch.
 18. The systemof claim 17, wherein the processing logic is further configured todetermine a total force associated with the first touch and the secondtouch, and wherein the processing logic is further configured todetermine a centroid location associated with the first touch and thesecond touch.
 19. The system of claim 18, wherein the processing logicis configured to determine the first magnitude of the first componentforce and a second magnitude of the second component force aredetermined based, at least in part, on a lever balance equation.
 20. Thesystem of claim 18, wherein the processing logic is configured todetermine the first magnitude of the first component force and a secondmagnitude of the second component force based on a system of equations.